This invention relates generally to the field of tritium isotope recovery from water and more specifically to a process for tritium removal from water by transfer of tritium from water to an elemental hydrogen stream, followed by membrane diffusion tritium stripping and enrichment, and final tritium enrichment by thermal diffusion. Several large scale facilities have been built in Canada, France, and more recently South Korea, to extract tritium from heavy water moderator systems for nuclear reactors. Kalyanam and Sood, “Fusion Technology” 1988, pp 525-528, provide a comparison of the process characteristics of these types of systems. Similar although smaller light water tritium recovery systems have been designed for fusion applications (see H. Yoshida, et al, “Fusion Eng. and Design” 1998, pp 825-882; Busigin et al, “Fusion Technology”, 1995 pp 1312-1316; A. Busigin and S. K. Sood, “Fusion Technology” 1995 pp 544-549). All current large scale systems employ a front-end process to transfer tritium from water to elemental hydrogen, followed by a cryogenic distillation cascade to perform all or most of the hydrogen isotope separation. Large scale membrane (gaseous) diffusion systems have been designed and built for uranium isotope separation. A thorough description of gaseous diffusion technology is provided by M. Benedict, T. Pigford and H. Levi, “Nuclear Chemical Engineering”, McGraw Hill (1981). Gaseous diffusion has never been used for large scale hydrogen isotope separation.
Thermal diffusion columns have been used to separate hydrogen isotopes on a small scale since the 1950's as described by G. Vasaru et al, “The Thermal Diffusion Column”, VEB Deutscher der Wissenschaften, Berlin, 1968. The use of this technology has been limited because it is not scaleable to large throughputs.
All current large scale processes for water detritiation are based on transfer of tritium from water to elemental hydrogen by: (a) a catalytic exchange reaction such as DTO+D2<⇄D2O+DT; (b) direct electrolysis of water, i.e., DTO→DT+½O2; or (c) water decomposition by a suitable reaction such as the water gas shift reaction: DTO+CO→DT+CO2. (See Kalyanam and Sood “Fusion Technology” 1988, pp 525-528; A. Busigin and P. Gierszewski, “Fusion Engineering and Design” 1998 pp 909-914; D. K. Murdoch et al, “Fusion Science and Technology” 2005, pp 3-10; K. L. Sessions, “Fusion Science and Technology” 2005, pp 91-96; J. Cristescu et al, “Fusion Science and Technology” 2005, pp 97-101; I—R. Cristescu et al, “Fusion Science and Technology” 2005, pp 343-348.)
Recently a Pd/Ag membrane cascade has been proposed as an alternative technology to cryogenic distillation for application to ITER. (D. L. Luo et al, “Fusion Science and Technology” 2005, pp 156-158). However, the hydrogen throughput of the proposed device was a factor of 1000 times smaller that in a CANDU reactor moderator water detritiation systems such as the Darlington Tritium Removal Facility. This alternative is feasible for a small degree of isotope separation such as upgrade of plasma exhaust gases containing approximately 50% deuterium and 50% tritium, to a concentration of 90% tritium suitable for fusion fuel recycling. The high tritium throughput for a large fusion device makes use of a small throughput technology such as thermal diffusion impractical. In a typical water detritaition application for a nuclear reactor the tritium throughput is miniscule by comparison to an ITER scale fusion machine, however the quantity of water to be processed is very large.
The prior art large scale hydrogen isotope separation cryogenic distillation process has the following drawbacks:    1. handling of liquid cryogens with associated hazards, such as high pressure potential upon warmup and evaporation; thermal stresses due to very low temperature process conditions; requirement for a vacuum insulated coldbox vessel to contain the cryogenic equipment;    2. large liquid hydrogen (and tritium) inventory, mostly tied up in distillation column packing;    3. potential for blockage of process lines due to freezing of impurities;    4. complex and costly process plant;    5. complex operation and maintenance;    6. non-modular process making it difficult to upgrade and to keep equipment spares;    7. requires batch operated dryers and a liquid nitrogen adsorber to purify feed to the cryogenic distillation cascade.
Large scale membrane diffusion has not been used in the past for hydrogen isotope separation due to a combination of commercial unavailability and the fact that enriching tritium from a few parts per million to 99+% purity requires a large number of discrete compression stages. To be competitive with cryogenic distillation, the number of compression stages needs to be reduced, especially at the high tritium concentration end of the process where tritium materials compatibility and safety issues exist.
Thermal diffusion has been used successfully for small scale tritium separation, even up to 99+% tritium, but cannot be easily scaled for large throughput. This is because thermal diffusion columns must operate in the laminar flow regime, and scale-up would push column operation into the turbulent flow regime (R. Clark Jones and W. H. Furry, “Reviews of Modern Physics”, 1946, pp 151-224). The alternative of constructing many small thermal diffusion columns in parallel is unattractive when the throughput requirement is large. Thermal diffusion columns also have low thermodynamic efficiency, which while unimportant at small scale becomes problematic at large scale.